We have carried out first-principles calculations to explore reaction mechanisms of the 2ereduction of CO2 to HCOOH or CO in photochemical reactions catalyzed by anatase (101) surface. Two energetically competitive reaction pathways to HCOOH were identified, which involve initial 1e (via bidentate) and 2e (via monodentate) reduction steps, respectively. The pathways of producing CO were also explored. From the electronic structure analysis, we have shown the role of the anatase surface in facilitating electron and proton transfer in CO2reduction. Based on the determined rate-limiting step, we have carried out screening of substitutional surface cation doping and found metallic elements that could substantially lower the reaction barriers. A simple model describing the relationship between the activation barriers and the binding energies of CO2− to the dopant surface site is proposed.